Reversal photoconductographic processing



July 13, 1965 F. URBACH REVERSAL PHOTOCONDUCIOGRAPHIC PROCESSING Filed 001;. 25. 1960 Franz Urbach I N V EN TOR.

United States Patent 3,194,748 REVERSAL PHUTOCONDUCTOGRAPl-HC PROCESSING Franz Urhach, Rochester, N. assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey Filed Oct. 25, 1960, Ser. No. 64,903 4 Claims. (Cl. 204-18) The present invention relates to photoconductography.

Photoconductography forms a complete image at one time or at least a nonuniform part of an image as distinguished from facsimile which at any one moment produces only a uniform dot. The present invention relates particularly to the formation of a direct positive image either on the photoconductive layer or on a separate re ceiving sheet.

Cross reference is made to the following series of applications filed July 28, 1960.

Serial No. 45,940, John W. Castle, Jr., Photoconductognaphy Employing Reducing Agents.

Serial No. 45,941, Raymond F. Reithel, Photoconduc- .tolithography Employing Nickel Salts, now abandoned, now continuation-inpart Serial No. 120,863, filed June 7, 1961, now US. Patent No. 3,106,157.

Serial No. 45,942, Raymond F. Reithel, Photoconductolithography Employing Magnesium Salts, now US. Patent 3,053,179.

Serial No. 45,943, Raymond F. Reithel, Photoconductography employing Spongy Hydroxide Images, now abandoned, now continuation-impart Serial No. 120,035, filed June 27, 1961, now US. Patent No. 3,106,518.

Serial No. 45,944, Raymond F. Reithel, Method for Making Transfer Prints Using a Photoconductographic Process.

Serial No. 45,945, Raymond F. Reithel, Photoconductognaphy Employing Manganese Compounds.

Serial No. 45,946, Raymond F. Reithel, Photoconductography Employing Molybdenum or Ferrous Oxide, abandoned, now continuation-in-part Serial No. 120,- 036, filed June 27, 1961, now US. Patent No. 3,106,156.

Serial No. 45,947, Raymond F. Reithel, Photoconduc- .tography Employing Cobaltous or Nickelous Hydroxide, abandoned, now continuation-impart Serial No. 120,- 037, filed June 27, 1961, now US. Patent No. 3,057,788.

Serial No. 45,948, Donald R. Eastman, Electrophotolithography.

Serial No. 45,949, Donald R. Eastman, Photoconductolithog-raphy Employing Hydrophobic Images, now US. Patent 3,152,969.

Serial No. 45,950, Donald R. Eastman and Raymond F. Reithel, Photoconductography Employing Electrolytic Images to Harden or Soften Films, now US. Patent 3,106,516.

Serial No. 45,951, Donald R. Eastman and Raymond F. Reithel, Ph-otoconductograp'hy Employing Absorbed Metal Ions, now abandoned, now continuation-in-part Serial No. 120,038, filed June 27, 1961.

Serial No. 45,952, Donald R. Eastman and Raymond F. Reithel, Photocond-uotography Employing Spongy Images Oontaining Gelatin Hardeners, now US. Patent 3,028,147.

Serial No. 45,953, John .T. S'agura, Photoconductography Employing Alkaline Dye Formation, now US. Pat ent 3,057,787.

Serial No. 45,954, John I. Sagura and James A. Van Allan Photoconductography Employing Quaternary Salts.

Serial No. 45,955, Franz Urbach and Nelson R. Nail, Uniform Photoconductographic Recording on Flexible Sheets, now abandoned.

Serial No. 45,956, Franz Urbach and Nelson R. Nail,

ice

High Contrast Photoconductographic Recording, now abandoned.

Serial No. 45,957, Nicholas L. Weeks, Photocon-ductography Involving Transfer of Gelatin, now US. Patent 3,103,875.

Serial No. 45,958, Donald R. Eastman, Photoconductolithography Employing Rubeanates, now US. Patent No. 3,095,808.

Serial No. 45,959, Donald R. Eastman and Raymond F. Reithel, Electrolytic Recording with Organic Polymers, now US. Patent 3,106,155.

Serial No. 46,034, Franz Urbach and Donald Pearl- 'man, Electrolytic Recording, now abandoned.

Cross reference is also made to an application Serial No. 109,977 of Donald P earlman entitled Direct Positive Ihotoconductography filed May 15, 1961.

Cross reference is also made to cofiled applications SN. 64,901 Franz Urbach and SN. 64,902 Franz Urbach and Nelson R. Nail.

Photoconductography is described in detail in British 188,030 Von Brook and British 464,112 Goldmann, modifications being described in British 789,309 Berchtold and Belgium 561,403 Johnson et al.

In order to obtain reversal processing according to the present invention certain unusual electrolytic phenomenon are utilized and certain others must be overcome.

Conducting recording materials of the type described in application Serial No. 45,953 mentioned above, are substantially colorless at one pH but become colored at a different pH, for example at :a higher pH. Change in pH at the cathode or, in some cases .at the anode has accordingly been used to record photoconductographic images. For example a dye precursor which is colorless at a relatively low pH may form a dye when the pH is raised to a certain value and this increase in pH may be provided by a p'hotoconductive layer acting as a cathode. The present invention further requires that the conductorecordin-g material either changes in pH in the opposite direction (in the example given, the change is toward lower pH) at the anode or the material must be such that, when it is adjacent to an anode during the passage of electrolytic current, it becomes less susceptible to being changed to high pH when subsequently placed near a cathode.

In the dye precursor example, passage of cur-rent in such a direction that the dye precursor is adjacent to an anode actually lowers the pH slightly and therefiore in a subsequent step in which it is adjacent to a cathode, it requires greater intensity or time of passage of current to raise the pH to the dye forming value.

Thus the present invention is limited to those photoconductographic processes which depend on the polarity of the direct current, i.e. on whether the recording layer is in contact with a cathode or an anode.

A special problem comes up in the case of zinc oxide in resin since this type of photoconductor is known not to act as an anode when in contact with a liquid electrolyte. A simple manner of overcoming this incidental problem is discussed later in the specification.

A photoconductographic sheet consists of a photoconductive layer in electrical contact (directly or through a unidimensional conducting layer) with a conductorecording or electrosensitive layer, often referred to as an electrolyte. The recording layer may be integral with, or on a separate support from the photoconductive layer. Sensitivity of the photocond-uctive layer to incident radiation, sensitivity of the recording layer to the passage of electric current and the electrical potential applied for development all affect the ultimate print density and contrast.

The present invention consists essentially of four steps (1) image exposure (2) desensitizing (3) uniforming and (4) development. Steps 1 and 2 may be performed- 3,1 3 simultaneously; steps 3 and 4 are usually performed simultaneously.

The first step (image exposure) is a standard one; it may be by contact printing,'projection printing or area. scanning. There is'not'hing unusual in this step. It creates or produces variations in conductivity distributed ima-gewise across the photocond uctive layer. In some photoconductors these variations cease when the image illumination is turned off and in this case the second de'sensitizing step must take place simultaneously with the exposure step. In other photoconductors, such as the usual zinc oxide in resin, the variations in conductivity persist and the desensitizing step may be applied at least partly subsequent to the exposure step.

The second step (desensitizing) is an unusual one. It is obtained by applying D.C. current whose polarity is opposite to that normally used for development. It depends on the fact that certain electrolytes which are normally developed (photoconductographically speaking) at the cathode have been found to be desensitized at the anode of an electrolytic system. As a specific example, it is pointed out that the pH of an electrolytic bath generally increases at the cathode and falls at the anode when current passes. Certain photoconductographic processes depend, for the production of the image in the recording layer, on such increase in pH at the cathode. According to the present invention, the imagewise distribution of variations in conductivity produced by the exposure step are used to control the application of electric current to the recording layer in contact with an anode. The image may be considered as made up of light areas and dark areas. The light areas of the exposure image correspond to the conducting areas of the photoconductive layer and these in turn correspond to the desensitized areas or areas of lowered pH in the recording layer. second step, the recording layer consists of areas of normal sensitivity (unchanged pH in the example given) corresponding to the dark image areas and clesensitizied (lower pH) areas corresponding to the light areas of the image.

The third step involves the elimination of the variations in conductivity. The uniforming of the conductivity is most conveniently obtained either by replacing the photoconductive layer with a uniform metal electrode or by flooding the photoconductive layer with light so that it is uniformly conducting. The uniformity may extend over the whole image area or one or both electrodes (in step 4 discussed below) may be rollers which move across the recording sheet to apply a uniform current.

The fourth step which is applied when the conductivity of the system has been rendered effectively uniform, .is similar to the normal photoconductographic development step, but produces a direct positive instead of a negative. In the example given a uniform current passed through the electro-recording layer in contact with a cathode raises the pH more or less uniformly throughout the layer. However, the light image areas have a lower pH than the Thus after the uniformly, the dark areas reach a critical value before the light areas. This critical value is the one at which darkening or density is produced. For example the recording layer may contain a dye precursor which is color less at the pH thereof before the present process is applied, but which forms a dye at a certain higher value of the pH. The fourth step raises the pH of the dark image area above this certain value, but is terminated before it raises the pH of the light image areas to this value.

The first DC. current applied at the anode must of course be of sufficient time and intensity, i.e. in sufficient amount, to appreciably lower the pH. The process becomes too critical for commercial handling if the lowering of the pH is less than 0.1 unit, say. The final direct current applied at the cathode must similarly be in sufficient amount to raise the pH to the critical value in the dark image areas but not to raise the pH in the light image areas quite to this critical value.

The operation of the invention andits advantages will be more fully understood when read in connection with the accompanying drawing in which: 1

FIG. 1 is a flow chart schematically illustrating a preferred embodiment of the invention. 7

FIG. 2 similarly illustrates an alternative embodiment employing a zinc oxide photoconductor.

In FIG. 1 a positive transparency 1% is illuminated by a lamp 1i and an image of the transparency is focused by a lens 12 on a photoconductive layer 13 carried on a conducting support'lt which actsas one electrode in the overall system. i

In contact with the photoconductor 13 is a recording layer 2% (which may be liquid) containing a'material which becomes colored. when the pH is raised. A transparent counter electrode 21 consisting of glass with a metalized surface constitutes the other electrode of the system and is in contact with the recording layer 20, which may include a conducting support such as paper next to the electrode 21 or which may be integral with the photoconductor 13.

DC. potential is applied from a source indicated sche: matically at 22 through a polarity reversing switch 23. During the imagewise exposure of the photoconductive layer 13, the switch is positionedas shown so that the photoconductor 13 is the anode and the counter-electrode 21 is the cathode. The resulting current flows through the system (in the imagewise. exposed areas of the photo conductor 13) so as to lower the pH of the layer 20 at the interface between the layers 26 and 13, in the areas corresponding to the exposed areas of the photoconductor 13. Accordingly the recording material or dye precursor remains colorless throughout this step of the process. Howeventhe light areas of theimage produce areas of lower pH in the recording layer 20.

The imagewise exposure is then terminated and the material is moved under a lamp 25 and reflector'26 which uniformly illuminate the photoconductive layer 13 cansing it to be uniformly conducting. The switch 23 is moved to the. other position so as to reverse the polarity and now the photoconductor 13 is the cathode and the current flowing between it and the counter-electrode 21 through the layer 20 tends to raise the pH of the layer 20 at the interface This current is continued long enough to raise the pH in the area corresponding to the dark areas of the original image, up to thevalue at which a dye is formed. However, the current is cut off before it raises the pH of the other areas to this value. A positive image is produced on the photoconductive layer 13 and the image has good definition, density and contrast.

The exposure and electrolyte treatment steps are combined at each stage of the arrangement shown in FIG. 1. However, if the conductivity persists after the exposure is cut off, the electrical potential may be applied after each stage of exposure. This possibility 'is, in practice, complicated when zinc oxide in resin is used as the photoconductor. This material has excellent image persistence qualities but when it is in contact with a liquid, for example a liquid electrolyte, there is apparently some surface effect which prevents the zinc oxide layer from acting as an anode. Itwill act as a cathode, however, and when an intermediate layer of metal is present, it can be used as an anode.v

An alternative embodiment is shown in FIG. 2. Zinc oxide in resin'photoconductive layer 34 is coated on a support which is a unidimens ional conducting layer 40. A unidimensional conductor is one which conducts electricity through the thickness of the sheet but not laterally. Such sheets are described in the Berchtold patent mentioned above; The front of' the photoconductor is then provided wtiha transparent metal layer 35, preferably by evaporationin a vacuum. The edges of this metal layer are in electrical contact .witha thick metal ring or strip 37 to which wire may be later attached. In the arrangement shown, exposure is through the transparent electrode since most unidimensional conductors are more or less opaque or have an effectively high density. A positive transparency 30 is moved in front of a lamp 31 as indicated by the arrow 32. An image of the transparency 30 is focused by a lens 33 on the photoconductor 34 which is moved as indicated by the arrow 36, synchronously with the image focused thereon.

After exposure, the unidimensional layer 40 is placed in contact with an electrolyte 41 carried on a paper support 42. This in turn is placed on a sheet of blotting paper 43 which is wet with a potassium chloride solution, so that the cathodic reaction (between electrodes 37 and 44 when current is supplied from a source of potential indicated schematically at 45) takes place at the cathode 44. The electrolyte 41 is in contact with an anode 40. Since the zinc oxide layer 34 is in electrical contact with the unidimensional conductor 40 but not in contact wtih the liquid electrolyte 41, the surface phenomenon of zinc oxide which prevents it from acting as a cathode is not present. The electrolyte 41 has a certain pH before current is passed therethrough. The effect of the current corresponding to the light areas of the image is to reduce this pH at the interface between the unidimensional layer 40 and the electrolyte 41.

The receiving sheet consisting of the electrolyte 41 and its paper support 42 is then passed between electrode rollers 50 and 51, supplied with current from a source of potential 52 so that the electrode 50 acts as the cathode. There is no imagewise distribution of conductivity in this case since the rollers 50 and 51 apply a uniform potential as they roll across the sheet. The surface of the layer 41 in contact with the cathode 50 increases in pH until the dark areas pass the critical value at which a dye forms. The current is then cut off and one has a finished print which may be washed 011?, if desired, to remove any unused electrolyte.

Example 1 This example is similar to that shown in FIG. 2 except that the blotting paper 43 was omitted and the back of the sheet 42, itself, was moistened with potassium chloride solution as an electrolyte. Also the desensitizing current was applied during exposure. Preparation of the recording sheet 41, 42 was as follows:

0.1 gram of p-N-morpholino-benzene-diazonium zinc chloride was dissolved in 25 cc. of a 6% gelatin solution containing 0.2 gram potassium chloride and 0.75 cc. of glycerol. The pH of this solution was adjusted to 3.0 with a stabilizer solution consisting of 1.0 gram thiourea, grams boric acid, and 10 grams tartaric acid dissolved in 250 cc. of distilled water. The pH was then further adjusted to 2.3 with sulfuric acid. 0.1 gram of 3-hydroxy-B-hydroxyethyl-2-naphthamide was dissolved in 5 cc. of methyl alcohol and added to the above solution warmed and with stirring. This mixture was then filtered through a balloon silk filter bag and coated 0.010 of an inch on gelatin sized and ferrotyped paper base and dried on a ferrotype board.

The unidimensional conductor was made by winding a very large coil of insulated copper wire onto a 4-inch diameter drum and impregnating the winding with epoxy resin insulator while the winding was proceeding. The total diameter of the drum was l2-in. This drum was then sliced up radially making very thin sheets of unidimensional conducting material, the surfaces of which were polished. A large number of other methods of making unidimensional conductors are known but this happens to be the one used in this particular example. There were about 15,000 turns per square inch of wire and hence about 15,000 conductors per square inch embedded in the unidimensional conducting sheet. In this particular example a photoconductive layer of cadmium sulfide in a resin binder was coated on the unidimensional layer and then overcoated with a transparent metal electrode by evaporation. The material was then exposed as illustrated in FIG. 2 except that the desensitizing step took place at the same time as the exposing step. That is, the unidimensional layer was in contact with the above diazo layer during exposure and the unidimensional layer acted as an anode. That is, the first two steps illustrated separately in P16. 2 were performed at the same time. The above described diazo paper was moistened (the rear surface thereof) with potassium chloride solution as an electrolyte and an aluminum foil counterelectrode (corresponding to the electrode 44 in FIG. 2) was placed against this moistened surface and rolled into contact with a rubber roller. A positive image was projected onto the photoconductor through the transparent coating thereon using 165 ft. candle illumination for 25 seconds. During this 25 seconds exposure, the photoelectrode (consisting of layers 34, 35 and 40 of FIG. 2) was made the anode and the aluminum foil on the back of the recording paper was made the cathode with volts potential being applied during the 25 seconds exposure. The exposure was then cut oif and the polarity was changed so that the photoelectrode was negative and the aluminum foil positive. At the same instant, the photoelectrode was illuminated from the back by an overall uniform intensity of ft. candle illumination for 25 seconds. A direct positive print on the diazo paper resulted. A large number of prints were made in this manner with slight variations in the various times of exposure and percentage of materials used. Excellent results were obtained.

Many of the copending applications mentioned above utilize materials which change color when the pH is raised and these materials could be used directly as examples of the present invention, providing the peculiar (desensitizing and uniforming) steps of the present invention are applied thereto.

Having thus described examples of my invention I wish to point out that it is not limited thereto but is of the scope of the appended claims.

I claim:

1. A reversal photoconductographic process comprising illuminating by an image having light and dark areas a substantially colorless photoconductive layer to produce an imagewise distribution of variations in conductivity, passing D.C. current similarly distributed through the layer in contact with a substantially colorless dye precursor which forms a dye when the pH is raised to a certain value, the DC current being in the direction and of suificient quantity to lower the pH of the substantially colorless dye precursor in the light image areas at least .1 unit leaving it still substantially colorless, terminating the imagewise illuminating and then uniformly illuminating the photoconductive layer and passing a second DC. current substantially uniformly distributed through the layer in the opposite direction and of sufficient quantity to raise the pH of the dye precursor in the dark image areas to said certain value to form said dye in said dark image areas without raising it to such value in the light image areas.

2. A reversal photoconductographic process comprising exposing a zinc oxide in resin photoconductive layer to produce an imagewise distribution of variations in conductivity, placing the exposed layer while the variations persist in contact with the unidimensional conductor the other side of which is in electrical contact with a substantially colorless dye precursor which forms a dye when the pH is raised to a certain value, passing D.C. current through the conducting areas of the photoconductive layer and through the adjacent areas of the unidimensional conductor and the dye precursor in the direction and of sufficient quantity to lower the pH of the dye precursor in said areas leaving it still substantially colorless and then passing through the dye precursor, substantially uniformly distributed DC. current in the other direction and of sufficient quantity to raise the pH of the dye precursor to said certain value to form said dye in other than said areas without so raising it to said certain value in said areas. 7

3. The process according to claim 2. in which during the second DC. current passing step, the photoconductive la er and unidirnensional layer remain in contact with the when the pH thereof is raised to a certain value, the

pH of the layer being below said value so that the layer is and remains substantially colorless before and during said passing, said DC. current being in the direction and of sufficient quantity to lower the pH of the current passing areas of the recording layer at least 0.1 unit,

then apply a uniformly distributed DC. potential in the other direction and by said potential, passing through the recording layer a substantially uniformly distributed DC. current of 8 suflicient quantity to raise the pH of the higher pH areas of the recording layer to said certain value to darken said higher pH areas Without raising the pH of the lower pH areas to this value.

References fiited by the Examiner UNITED STATES PATENTS 2,339,267 1/44 Hogan et al. 204-2 2,692,178 10/ 54 Grandadarn 96-1 2,764,693 9/56 Jacobs et al. 96--1 2,866,993 12/58 Berchtold 96-1 2,397,037 7/59 Tolf 204-2 2,927,834 3/60 Solar 204-2 2,953,335 12/60 Hall et al. 961 3,010,883 11/61 Johnson et al 20418 3,057,787 10/62 Sagura 204-18 FOREIGN PATENTS 4/37 Great Britain.

OTHER REFERENCES Greig: ,Proc; of I.R.E., October 1948, pages 1224-35.

JOHN H. MACK, Primary Examiner. t

PHILIP E. MA-NGAN, JOSEPH REBOLD, Examiners. 

1. A REVERSAL PHOTOCONDUCTOGRAPHIC PROCESS COMPRISING ILLUMINATING BY AN IMAGE HAVING LIGHT AND DARK AREAS A SUBSTANTIALLY COLORLESS PHOTOCONDUCTIVE LAYER TO PRODUCE AN IMAGEWISE DISTRIBUTION OF VARIATIONS IN CONDUCTIVITY, PASSING D.C. CURRECT SIMILARLY DISTRIBUTED THROUGH THE LAYER IN CONTACT WITH A SUBSTANTIALLY COLORLESS DYE PRECURSOR WHICH FORMS A DYE WHEN THE PH IS RAISED TO A CERTAIN VALUE, THE D.C. CURRENT BEING IN THE DIRECTION AND OF SUFFICIENT QUANTITY TO LOWER THE PH OF THE SUBSTANTIALLY COLORLESS DYE PRECURSOR IN THE LIGHT IMAGE AREAS AT LEAST .1 UNIT LEAVING IT STILL SUBSTANTIALLY COLORLESS, TERMINATING THE IMAGEWISE ILLUMINATING AND THEN UNIFORMLY ILLUMINATING THE PHOTOCONDUCTIVE LAYER AND PASSING A SECOND D.C. CURRENT SUBSTANTIALLY UNIFORMLY DISTRIBUTED THROUGH THE LAYER IN THE OPPOSITE DIRECTION AND OF SUFFICIENT QUANTITY TO RAISE TH PH OF THE DYE PRECURSOR INTHE DARK IMAGE AREAS TO SAID CERTAIN VALUE TO FORM SAID DYE IN SAID DARK IMAGE AREAS WITHOUT RAISING IT TO SUCH VALUE IN THE LIGHT IMAGE AREAS. 